The invention relates to a method for changing from a coasting or free-rolling mode of a motor vehicle with an internal combustion engine and automatic clutch, which is disengaged during the coasting or free-rolling mode, to a fuel cut-off mode with the clutch engaged, according to the preambles of the independent claims.
In motor vehicles with an internal combustion engine and an automatic clutch, an automatic changeover to coasting mode is known if the driver is not requesting propulsion, i.e. if he is not operating the gas pedal. Coasting is a state in which the drive train is open by automatic disengagement of the clutch and the internal combustion engine continues to run in neutral. As the drag torque of the internal combustion engine is eliminated, fuel can be saved. If the driver operates the vehicle brakes, however, the conventional behavior of motor vehicles without an automatic clutch is preferred, whereby the motor vehicle operates in the fuel cut-off mode.
Fuel cut-off, which is also used for fuel saving, means an interruption of the fuel supply to the internal combustion engine, so that the latter does not output any power, but is only kept running by the inertia of the vehicle, i.e. is pushed by the vehicle with the clutch engaged, wherein the internal combustion engine acts as a brake and the motor vehicle experiences a deceleration because of the drag torque of the internal combustion engine.
A so-called free-rolling mode or Roll-Start/Stop mode (RSS) represents an alternative to a coasting mode, wherein the fuel supply to the internal combustion engine is completely cut off. The drag torque of the internal combustion engine is also eliminated in this case. Both mentioned operating modes, coasting mode and free-rolling mode, are possible with all drive trains, which can be opened automatically in some way. Of course this applies to automatic transmissions (conventional torque converter or dual clutch gearboxes), but also to automatic manual transmissions (AMT), continuous variable transmissions (CVT) and manual gearboxes that are fitted with an automatic clutch, i.e. an electrically or electronically operated clutch. Within the scope of this disclosure, the term coasting mode or free-rolling mode should also include associated phases in which the internal combustion engine is temporarily operated at other revolution rates than the idling revolution rate with the clutch disengaged.
A generic method is also known from DE 102 21 701 A1, whereby the change from coasting or free-rolling mode to fuel cut-off mode takes place relatively smoothly in order to remain almost unnoticed by the driver. However, it should certainly be noticed in this case, as the driver should get feedback about the change of the vehicle state.
The object of the invention is to make a change from coasting or free-rolling mode to fuel cut-off mode more comfortable for the driver.
This object is achieved by a method and by a device with the features of the independent claims. Advantageous developments of the invention are specified in the dependent claims.
The invention is based on the knowledge that there are driving situations in which the driver does not expect feedback about the change from coasting or free-rolling mode to fuel cut-off mode, but is rather disturbed or irritated thereby. This can be prevented by the invention.
Such a driving situation is e.g. that in which a motor vehicle in the coasting or free-rolling mode is travelling behind another motor vehicle that is travelling with fuel cut-off, e.g. because no coasting or free-rolling mode is provided therein. If the driver of the motor vehicle travelling in the coasting or free-rolling mode is expecting e.g. that the vehicle in front will decelerate in a conventional manner when approaching a red light, the coasting or free-rolling mode will not normally be terminated as long as the driver only operates the brake lightly. But if the driver of the vehicle travelling in the coasting or free-rolling mode is surprised by the behaviour of the vehicle travelling in front, he has to operate the brake so strongly that the coasting or free-rolling mode is automatically terminated. In this case it is disturbing for the driver to feel the change to the fuel cut-off mode and to experience the sudden additional braking effect connected therewith.
Another such driving situation is e.g. that in which a motor vehicle in the coasting or free-rolling mode is currently driving down a downhill road. The coasting or free-rolling mode should also be automatically terminated then, but it is irritating for the driver to feel the change to the fuel cut-off mode.
The vehicle electronics that control the operation of the engine, clutch and gearbox are capable of automatically identifying driving situations of both of the above-mentioned types, i.e. those of the first described type in which a braking demand on the part of the driver is detected during the coasting or free-rolling mode, whereby the braking torque of the vehicle brake exceeds the currently available braking torque of the internal combustion engine, and those of the latter described type in conditions that are independent of pedal operations by the driver, e.g. if sensors or navigation devices of the motor vehicle identify that the motor vehicle is currently driving down a downhill road. In both cases, with the invention the increase of the drag torque of the internal combustion engine acting on the motor vehicle during the changeover from coasting or free-rolling mode to fuel cut-off mode is made so slowly and smoothly that the driver does not feel it.
This can be carried out in two ways as required. On the one hand the clutch can be engaged so slowly that the change of vehicle deceleration during the engagement of the clutch does not exceed a preset value. In addition or alternatively, after engagement or partially even during engagement of the clutch, the fuel supply to the internal combustion engine can be reduced so slowly that the change of vehicle deceleration during the changeover does not exceed the preset value.
In this case it can be advantageous for a motor vehicle with a multi-speed transmission for a dedicated preset value to be provided for each gear, which can also be selected from a plurality of preset values depending on the ratio between the braking torque produced by the brakes on the wheels and the maximum drag torque produced by the internal combustion engine.
The time that it takes for the drag torque of the internal combustion engine to increase from zero to one hundred percent during the changeover from coasting or free-rolling mode to fuel cut-off mode without the driver noticing this typically lies in a range of about 1 to 5 seconds.